1. Field of the Invention
The present invention relates to a composition for etching a double metal layer, and more particularly, to a composition for etching a double metal layer, a method of fabricating an array substrate, and a method of forming a double metal line for a liquid crystal display device.
2. Discussion of the Related Art
A size of a thin film transistor-liquid crystal display (TFT-LCD) device scales with a length of a metal line transmitting signals in a TFT-LCD device. Accordingly, a resistance of the metal line increases with increased TFT-LCD device size, and the increased resistance causes signal delay. To prevent the signal delay, a metallic material having a relatively low resistance has been suggested for the metal line. For example, aluminum (Al) alloy, molybdenum (Mo), molybdenum (Mo) alloy, copper (Cu), copper (Cu) alloy, silver (Ag) and silver (Ag) alloy have been suggested and researched.
A metal line in a TFT-LCD device is obtained by depositing a metallic material and patterning the deposited metal layer. The patterning step can be performed by a dry etching method or a wet etching method. In general, the metal layer is patterned by a wet etching method. In a wet etching method, a metal layer is patterned using an etching solution referred to as an etchant. The kind of etchant to be used is determined according to a metallic material for the metal layer.
Aluminum (Al) alloy is widely used due to its low resistance. However, aluminum (Al) alloy has a relatively low tolerance to chemicals, such as etchants. Moreover, since aluminum (Al) alloy has a low melting temperature, defects—such as hillocks—are generated in subsequent high temperature processes. Accordingly, a double metal layer including aluminum neodymium (AlNd) and molybdenum (Mo) has been suggested instead of a single metal layer of aluminum (Al) alloy.
A double metal layer of aluminum neodymium (AlNd) and molybdenum (Mo) may be etched using a single etchant to obtain a double metal line of aluminum neodymium (AlNd) and molybdenum (Mo). However, since an etch rate of molybdenum (Mo) is different than an etch rate of aluminum neodymium (AlNd), it is difficult to obtain a double metal line of aluminum neodymium (AlNd) and molybdenum (Mo) having a uniform profile. For example, when a double metal line of molybdenum/aluminum neodymium (Mo/AlNd) is formed from a double metal layer having a molybdenum (Mo) layer formed on an aluminum neodymium (AlNd) layer, the double metal line may have an overhang structure in places where the aluminum neodymium (AlNd) layer does not exist under some portion of the molybdenum (Mo) layer because of over-etching of the aluminum neodymium (AlNd) layer. The overhang structure causes an electrical short or an electrical opening of upper and lower metal lines in a subsequent process. To improve the overhang structure problem, an additional dry etching process is required. In addition, when aluminum neodymium (AlNd) has poor wettability a single etchant, aluminum neodymium (AlNd) residues may be left behind.
When a double metal layer is formed having a structure with an aluminum neodymium (AlNd) layer formed on a molybdenum (Mo) layer, a tail of molybdenum (Mo) remains at a periphery of a double metal line of aluminum neodymium/molybdenum (AlNd/Mo) formed from such a double metal layer because of under-etch of the molybdenum (Mo) layer.
FIG. 1 is a scanning electron microscope (SEM) image showing a double metal line obtained through an etching process according to the related art. While fabrication steps are not shown in FIG. 1, a double metal line of molybdenum/aluminum neodymium (Mo/AlNd) is formed by etching a double metal layer of a lower molybdenum (Mo) and an upper aluminum neodymium (AlNd). After the molybdenum (Mo) layer and the aluminum neodymium (AlNd) layer are sequentially formed on a substrate, a photoresist (PR) pattern is formed on the aluminum neodymium (AlNd) layer. The double metal layer is etched using the PR pattern as an etch mask to form the double metal line, and then the PR pattern is removed. FIG. 1 shows the double metal line after removing the PR pattern. In FIG. 1, the lower molybdenum (Mo) layer remains at a peripheral region “A” of the double metal line to form a molybdenum (Mo) tail. When such a double metal line is used for a display device, the molybdenum (Mo) tail causes a stain in displayed images of the display device.